This invention relates to an electrically controlled display device for generating heat in a selected patern in order to display information.
Several different technologies are used today to present information. These can be grouped in two major categories electronic and mechanical. Older mechanical displays are still in use today in many places. They are however slow since each message needs to be flipped up to the display surface and they are also inflexible as only a fixed and predetermined set of information can be displayed. Since they are mechanical and have numerous moving parts, they are also expensive to manufacture and to maintain.
Modern versions of these mechanical display exist. These modern versions have a large amount of pixels which can be arbitrarily rotated to show either the front side or the back side. Information is then displayed by addressing the desired pixels and flipping them. The drawbacks with this modern display are that they still contain a lot of moveable parts and that they are slow.
Public information displays based on arrays of light emitting diodes are common today. The display consists of a large amount of light emitting diodes which can be individually controlled to display information. Displays based on red or yellow light emitting diodes are common in many situations, and full colour displays in very large formats are used in sport stadions and the like. In a full color display, each pixel consists of at least three different diodes (red, green and blue). These displays can be made very large and are capable of displaying full motion video.
Displays using light emitting diodes to show information have excellent contrast and readability when ambient light is low or non existent. However they have very poor readability in sunlight and the information is only visible in a very narrow viewing angle.
Displays using liquid crystals to show information are suitable for protected enviroments but are not generally used outdoors due to their relatively low level of brightness, low contrast in bright sunlight and need for protection from the elements. Many liquid crystal displays have been prepared by applying a liquid crystal material onto a conductive electrode element, such as those described in U.S. Pat. Nos. 3,578,844 and 3,600,060. Such electrodes are used to generate an electric field across the liquid crystal layer and require complex electroding arrangements.
An another type of display is that which uses a reversible thermochromic material, i.e. one that changes color upon exposure to a temperature above a transition point. U.S. Pat. No. 3,354,565 describes the use of such a material to produce a passive information display. A metal iodide or metal bromide thermochromic material is desposited over an array of thin resistors positioned on an alumina substrate. The resistors are formed of barium titanate, a material exhibiting a positive temperature coefficient of resistance (PTC behavior). They are connected by suitable electrodes so that when power is applied to selected elements, specific resistors heat up above the transition point of the thermochromic material, causing it to change color and display information. The PTC effect prevents the resistors from overheating and damaging the display.
Displays comprising ceramic resistors such as barium titanate are subject to a number of problems. Such resistors are brittle and difficult to form or machine into intricate shapes or large sizes. They may be subject to cracking under high electrical or thermal stresses, particularly when exposed to overcurrent conditions. The resistors are difficult to manufacture into very thin elements, limiting the rate of thermal transfer to the thermochromic material. There is generally a mismatch in thermal coeffients of expansion between the ceramic and the thermochromic, which can result in cracking or delamination of the thermochromic from the ceramic resistor in use.
The international patent application WO 94/26528 discloses a display for the visualization of still or moving images where temperature changes are produced in a pattern of resistive elements and transformed to visible dots in a visualization medium. The display consists of a matrix having at least two sets of electrodes which sets cross each other and are distanced from each other by means of at least one resistive and/or inductive spacer means. The electrodes are arranged in rows and columns. The matrix if further connected to at least one control device for multiplexing, scanning or pulsating the control signals. The electrodes are individually addressable according to the electric signals from the control unit and arranged so that, when the selected electrodes in both sets of electrodes are energized, a temperature change is obtained at the cross point of the charged electrodes. This temperature change is conducted towards the display medium, which comprises a thermosensitive indication means, such as a layer, a body or the like. The surface of the display then transforms the temperature change to visible dots or the like.